The present invention relates to the production of hot melt adhesives, and more particularly to the production of hot melt adhesives containing additives such as pigments.
A wide variety of hot melt adhesive compositions are known and used in the art. Typically, hot melt adhesive compositions will include one or more polymers, tackifying resins, plasticizers and waxes. Sometimes other optional additives such as pigments, dyes, ultraviolet light absorbers, antioxidants, fluorescent agents, pH indicators and fragrances may also be added depending upon the desired end use of the adhesive. In addition to the above constituents, hot melt adhesives may also be customized to include blends of polymers, copolymers, resins, and like ingredients that provide specific desired physical properties, as is well known in the art.
The traditional method of preparing hot melt adhesives is to add resin, waxes and plasticizing oils to a jacketed tank or vessel equipped with a standard rotating vertical agitator. This mixture is then heated to a temperature in the range of about 250° F. to about 350° F. The polymer in the form of a bead, pellet, pillow, or other particulate form is then added to the hot resin/oil mixture, and the temperature of the vessel is maintained between about 300° F. to about 350° F. with continuous agitation. The composition is complete when the polymer is homogeneously blended into the resin/oil mixture. The time required for complete blending is usually 3–8 hours depending upon the polymer constituent and the degree of agitation. The polymer typically disperses very slowly into the resin/oil mixture resulting in relatively long periods of time where the polymer and other ingredients are subjected to a relatively high temperature which can cause degradation of the polymer as well as other ingredients.
Another common and commercially practiced method of preparing hot melt adhesive compositions involves initially preparing a taffy-like melt of the polymer in a kneader with subsequent addition of resin, oil or other ingredients. In this method, the polymer is initially heated and kneaded or worked into a taffy-like melt. Small amounts of resin and oil may also be initially added to facilitate the working of the polymer into the taffy-like melt. After about 0.5–2 hours, the final amounts of resin, oil and any other ingredients desired are added to the taffy-like melt.
In either of the above processes, the resultant liquid composition can be pumped from the processing equipment to a heated holding tank or the like. Thereafter, the adhesive composition is pumped into a container, such as a drum or the like, where it is allowed to cool and solidify for shipping. Alternately, the liquid composition may be pumped to pelletizing equipment to form pellets, pillows, ribbons, pastilles or other particulate configurations to be ultimately packaged in a bag for shipping. Hot melt adhesives are typically available commercially in various forms and packages, and a variety of different compositions, forms and packaging are available from many sources.
More detailed information on presently practiced methods for producing hot melt adhesives can be found in U.S. Pat. No. 5,498,646 and in the articles listed and referred to therein. Current technologies for manufacturing adhesives have several disadvantages, especially when formulating a composition that includes small amounts of additives such as pigments or dyes. Pigments and dyes are most often available in powder form. Since powdered additives do not melt or dissolve, they must be dispersed in the molten hot melt. It is difficult to thoroughly disperse powdered additives like pigments into high viscosity compositions such as hot melt adhesives because current technology typically utilizes mixing vessels with relatively low speed, low shear mechanical agitators, such as a rotating impeller immersed in the molten hot melt. Unless the additive is thoroughly blended therein, the resultant mixture has poor homogenation of the ingredients. If, for example, the additive is a pigment, poor homogenation results in inconsistent color of the resultant adhesive composition. Also, in order to obtain thorough mixing of an additive such as a pigment, the ingredients must be blended for relatively long times at high temperature which may cause degradation of the pigment itself. This in turn also results in inconsistent color. For these reasons, powdered additives are frequently dispersed first in another material such as mineral oil. This allows for more uniform dispersion in the molten hot melt.
Another disadvantage of the commercially practiced prior art methods, is that when it is desired to change the pigment, e.g. from blue to green or vice versa, or to a non-pigmented product, the entire system must be cleaned and flushed in order to prevent the next batch of adhesive from being tainted and off color. This results in relatively long down times and relatively high flush costs since the equipment must be completely purged of the prior pigment before charging the system with a new pigment or with a non-pigmented product.
Manufacturers of non-woven disposable articles such as disposable diapers, sanitary napkins, panty shields, surgical drapes, hospital pads, adult incontinence products and the like, are always desirous of reducing the costs of their product. One of the ways cost reduction can be achieved is by adding pigment to the hot melt adhesives which adhere the various substrates of the article together. One such substrate is typically composed of a thin layer of polyolefin material such as polyethylene or polypropylene film. Adding pigment to the hot melt adhesive allows the use of less expensive non-pigmented films since pigment in the adhesive permits reduced color concentration in the film which in turn results in the manufacturer being able to use a less expensive non-pigmented film. Also, the use of pigment in hot melt adhesives provides the manufacturer with the ability to combine the adhesive function with a visual enhancement function to improve the aesthetics of the manufactured article. Alternately, the pigmented hot melt can be used as a hot melt ink. Thus, improved methods of preparing adhesive compositions which avoid the disadvantages of conventional prior art methods is desirable.